Natural and synthetic thickeners are readily available and widely used in the art. It is well known that such thickeners, or as they are otherwise called, rheological additives, impart higher viscosity to solvent systems or water-based systems depending on the additive. Rheological additives generally are present only in small amounts, for example from 0.1% to 5%. Among these rheological additives are organoclays, fumed silica and organic and polymeric thickeners. Among the polymeric rheological additives are guar gum, tragacanth gum, pectin, xanthan and alginate (natural), hydroxyethyl cellulose, carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose (modified cellulosics) and polymers or copolymers of ethylenically unsaturated carboxylic acids and their derivatives, such as acrylic acid and acrylamide. The latter are ionic in nature and it has been shown that they may be used as flocculating and suspending agents for clay particles in water. These rheological additives are described in U.S. Pat. No. 4,738,726 issued to Pratt et.al., and Van der Watt and Bodman in Clay and Caly Materials, Volume 9, page 568 (1960).
Clay minerals are a group of minerals essentially composed of hydrated aluminum silicates with magnesium or iron proxying wholly or in part for the aluminum. Among these are montmorillonite, kaolin, illite, saponite, bentonite, hectorite, beidellite, stevensite, attapulgite and sepiolite. These minerals are described in Applied Clay Mineralogy, R. E. Grim, McGraw Hill, NY (1962).
One important type of clay mineral is hectorite, which is represented by the formula [(Mg.sub.6-x Li.sub.x)Si.sub.8 O.sub.20 (OH).sub.4-f F.sub.f ]x R.sup.+ wherein 0.57.+-..times..+-.1.15, f.+-.4 and R is selected from the group consisting of Na, Li, NH.sub.4 and mixtures thereof, which is obtained from various deposits. Hectorite deposits typically contain about 52% hectorite by weiqht, the other components being mainly calcite and dolomite and sometimes feldspar. Generally, the deposits are not used in the raw form since the impurities are believed to hinder the usefulness of the clay. For example, organoclays are typically produced by first removing the impurities and then reacting the purified clay to produce the organoclay. Hectorite clay which has been beneficiated, or treated, to improve the properties contains about 98% hectorite and requires several costly processing steps such as preparation of a water slurry, centrifugation and then separation of the purified clay, to convert the untreated clay deposits to concentrated or purified hectorite. Similarly, bentonite deposits contain about 5.8% quartz and it requires costly processing to convert bentonite deposits to the beneficiated form.
Thickeners and clay minerals often are combined in formulations for various products. For example, U.S. Pat. No. 4,657,943, issued to Wietsma, describes a thickening agent useful in textile padding and oil drilling muds containing an acrylate polymer and an aluminum silicate clay, montmorillonite clay or bentonite which shows a synergistic effect. The thickening agent has improved resistance under alkaline conditions and exhibits a synerqistic effect in water. However, the clays used in the thickening agent are chemically treated before being added to the other ingredients, which adds expense and complexity to the manufacture of these compositions.
Thickeners are used in a host of products such as paints, varnishes, enamels, waxes, oil drilling fluids, inks, adhesives, sealants, cosmetics, paper coatings, textile backings, bath emulsion systems, cleaners and polishers and detergents. One important use of thickeners is in joint compounds which are used extensively in the construction industry. The construction industry uses joint compounds and gypsum wallboard to construct walls and/or ceilings. Walls made in this method are constructed by fixing the panels to studs, and filling and coating the joints with a specially prepared material called joint compound. The process may include the application of a first coating, which when dried, is coated by a second joint compound layer. This layer may be sanded and a third layer may be applied and conventionally finished. A variety of joint compounds are used according to the various applications such as dry grade, taping grade, topping grade, all-purpose grade, ready mix grade and the like. Joint compounds typically are prepared in water-based solutions.
While joint compounds may contain a variety of compounds, fillers, binders and thickeners typically will be present. In U.S. Pat. No. RE29,753, issued to Williams, cellulosic materials and clay minerals are included in a joint compound formulation wherein the thickeners are listed as hydroxypropylmethyl cellulose, hydroxyethyl cellulose, alginates and sodium carboxymethylcellulose. Attapulgite clay is listed in the '753 patent as a non-leveling agent, while a modified clay composition is described as an anti-slip agent useful in the joint compound.
The modified clay composition mentioned in the '753 patent is named as Bentone LT, from NL Industries. The clay composition of Bentone LT is 50% spray-dried, refined or purified hectorite and 50% cellulosic materials. Bentone LT, a clay gellant, is described as a rheological agent in Davidson et.al., Paint and Varnish Production, July 1971. While this composition may be used as a rheological agent, it is expensive due to the treatment steps required to refine the hectorite.
In U.S. Pat. No. 4,558,079, issued to Desmarais, hydrophobically modified cellulosic materials were used as thickeners for water-based joint compounds. The joint compounds described also contain a resinous binder, mica and a clay compound. The hydrophobically modified cellulosic materials used as the thickener were chosen because they have less tendency to adsorb on the surface of clays. The clays used in the joint compounds described in the '079 patent are used to prevent sag. The thickeners described in the '079 patent, which are organic compounds, are costly due to various chemical processing steps and due to the use of the cellulosic thickeners as the only thickener in the joint compound.
U.S. Pat. No. 4,670,164, issued to Watson et.al., describes a thickening agent containing hydroxyethyl cellulose and an organoclay, along with a hydrocarbon. In addition to the requirement that the clay be treated with an organic compound, the patent describes adding an alcohol/ketone solution and the hydrocarbon in order to disperse the combination of hydroxyethyl cellulose and an organoclay to achieve the required viscosity in water. This thickening agent requires extra treatment of the clay which makes the agent more expensive and complicated to prepare.
Thus, there is a need in the art for a thickening composition useful in water-based systems which imparts increased viscosity at low shear rates in water-based systems. There also is a need in the art for a low cost thickener which can be easily utilized in various types of compositions.